While watching the first few scenes of “Juno,” Jason Reitman’s forthcoming film, it seems as though we should already know the score. Awkward, dysfunctional teen coming of age? Check. Surprise pregnancy? Check. The unlikely search for adoptive parents? Sounds about right.

But just as almost every adult in “Juno” is imbued with a complex identity that the film’s heroine has pegged all wrong, so too do we vastly underestimate what Mr. Reitman and his team of cinematic chemists have in store for the routine teen-angst formula. “Juneau, like the city in Alaska!” one adult after another proclaims with a smile. “No,” young Juno shoots back glibly, looking away in disdain.

And with each of her dismissive retorts, viewers feel their assumptions and expectations slowly being rerouted. Everywhere a cynic turns, ready to dismiss this story as another “Rushmore” rip-off or “Knocked Up” knock-off, there “Juno” is waiting.

Mr. Reitman shatters those conventions swiftly and mercilessly, thanks in large part to his secret weapon, a young actress who, in just two leading roles, has convinced more than a few observers that she may be one of the greatest talents of her generation. Ellen Page, a 20-year-old Canadian, has used her unconventional looks and her razor-sharp flair for sarcasm and understatement to subvert the notion of a leading lady. After appearing in more than a dozen films and TV shows, it was Ms. Page’s explosive performance in the 2005 role-reversal horror film “Hard Candy” that caught critics off guard. Playing a 14-year-old character with smarts to burn, Ms. Page succeeded in using her impressionable facade to toy with our sympathies, shifting gears from a fawning, precocious personification of impressionable youth to the tough, chiseled, asexual incarnation of revenge.

I attended one of the very first press screenings of “Hard Candy,” and I found myself duped, terrified for this unwitting teenager who not only agrees to meet her 32-year-old online stalker (Patrick Wilson) for coffee, but consents to drive with him to his house for a sexual rendezvous. It wasn’t until Ms. Page’s character (donning a red sweatshirt straight out of “Little Red Riding Hood”) drugged her attacker that the true incisiveness of her performance came into focus. As her character binds her abductor and proceeds to give him a lengthy, agonizing, on-screen castration, Ms. Page carefully takes her portrayal of the innocent victim and detonates it. The performance flipped the gender politics of the generic horror film.

Unlike some of her peers, such as Keira Knightley (in the new film “Atonement,” which opens next Friday), who have taken the more conventional path of playing sex objects and comic book heroes, hair blowing in the wind as the music swells, Ms. Page has gone the opposite route. Her black hair is kept short, her eyes don’t so much glint as glare, she usually dresses down, and her characters almost always call attention to their dearth of femininity.

In “Juno,” we meet Ms. Page’s title character as she stands in the front yard at dawn, chugging from a jug of orange juice as she hoses down one of the family’s comfy living room chairs. It was here, on this chair, where she lost her virginity and got pregnant, Juno tells us — less a life-changing event, she seems to think, than an inconvenient surprise. Sort of like a speeding ticket.

Not surprisingly, things have become awkward with her best friend Paulie (“Arrested Development’s” Michael Cera), and thanks to the pregnancy, Juno must deal with such annoying things as informing her parents, deciding whether to keep the child, and searching for adoptive parents interested in raising the baby. “You should’ve gone to China, you know, ’cause I hear they give away babies like free iPods,” she tells prospective parents Mark (Jason Bateman, equal parts suave and skeezy) and Vanessa (Jennifer Garner). “They pretty much just put them in those t-shirt guns and shoot them out at sporting events.”

Yet as the movie moves along, and as young Juno’s supposed ambivalence is replaced by a growing wave of genuine emotion, her impenetrable armor begins to crack. Working almost as the antithesis of her character in “Hard Candy,” who used her sweetness to sucker punch audiences with her vindictiveness, Ms. Page helps us to see in Juno the slow and painful journey of maturation.

While it’s a performance that may sound simple on paper, it has already bowled over some major industry watchers and pushed Ms. Page into the company of Meryl Streep and Laura Linney in this year’s Oscar debate. In September, at the Toronto Film Festival, it was enough to earn standing ovations from stunned crowds, and it led Roger Ebert to predict in his column that the movie will “win a best actress nomination” for Ms. Page. And that prediction has already begun to materialize: The night before “Juno” opens in Manhattan, a taped broadcast of last Tuesday’s Gotham Awards on NYC TV will show Ms. Page, a humble and dry-witted star still too young to drink, taking the stage as winner of this year’s “breakthrough actor” award.

ssnyder@nysun.com